A new map produced by NASA instruments pinpoints the need to monitor groundwater levels before aquifers run dry in the southern U.S.

Despite heavy rains this week, drought persists throughout much of the Midwest and Southern U.S., forcing residents to pump more water from deep underground. Now a new paper in Science points out that our nation's groundwater reserves are also being depleted throughout large swaths of the U.S., partly because we're not doing a good job of keeping track of how much groundwater we're extracting versus how much gets replenished through precipitation.

More From Popular Mechanics

"We don't really monitor water use," says James Famiglietti, an earth scientist at University of California, Irvine, and coauthor on the new paper. "It's crazy that we don't. It's like having a bank account and not keeping track of your deposits and withdrawals."

Famiglietti says NASA's Gravity Recovery and Climate Experiment (GRACE) satellites can help monitor groundwater on a global scale, and could even enhance flood and drought predictions.

The two GRACE satellites were launched in 2002 with the goal of mapping Earth's gravity field, which varies from place to place and over time. As the satellites follow each other around Earth orbit, navigating its lumpy magnetic field, they're constantly measuring their distance from one another. If the leading satellite moves through an area of high gravity, it gets pulled farther from the following satellite; that change in distance can be used to measure the gravity anomaly. As water gets redistributed all over the globe, it changes the distribution of mass and therefore the distribution of gravity around the planet, meaning GRACE can pick up on changes in ocean currents, runoff, snowmelt, aquifer levels and more.

Using data from GRACE, Famiglietti's team has created a comprehensive map of groundwater levels across the United States, and it reinforces the identification of some troubling trends. For one, the northern U.S. is getting wetter over time, leading to increased risk from floods, and the southern U.S. is getting drier, leading to heightened risk of drought. The map pinpoints six hotspots at high risk for water-related catastrophes: California's Central Valley, the Southern High Plains Aquifer in East Texas, and the areas around Houston, Alabama, and the mid-Atlantic states have all suffered steady groundwater depletion. Without proactive management, the authors say, aquifers in those regions could run dry within a few decades, putting the nation's food supply at risk. Meanwhile, water storage is increasing in the Missouri River Basin, making the region more prone to dangerous flooding.

"There's a picture that emerges from looking at these maps," Famiglietti says. "The subtext is that we have a national-scale problem that requires national-scale, coordinated, comprehensive water management . . . I don't think people realize the extent of the problem."

The U.S. Geological Survey compiles groundwater data from a variety of sources, but different wells are managed by different agencies that collect different measurements at different times. It can be difficult to put a full picture together from those data, especially since the majority of well data is either not recorded or not reported. "Currently we have a barebones network," says Kevin Dennehy, coordinator of the USGS's Groundwater Resources Program. "It's really a very small network, and we're relying on other agencies and state agency wells to try to get a picture on a national basis. We have detailed coverage in some of the areas, but there are sparse areas."

"There really aren't alternatives to GRACE in terms of allowing us to monitor water storage over entire regions or river basins," says Jeffrey McDonnell, a hydrologist with the University of Sasketewan in Canada. "It can expose the patterns and problems in a way that is very difficult to do from stitching together individual well observations."

Although GRACE is a unique tool for studying global water resources, at present it takes about a month to map groundwater levels, and it can only do so accurately at scales of 125,000 square miles or larger. As such, the data are meant to complement finer-scale measurements from individual wells. "If you're trying to make local decisions, you still need that detailed information," Dennehy says.

Of course, maintaining a steady water balance requires more than just data; the greatest challenges will be in changing human behavior. According to Famiglietti, that could include limiting local residents' water use in dry areas, restricting water-intensive industries, recycling sewage water, and ensuring that agriculture—which consumes 53.5 billion gallons of groundwater a day—is as efficient as possible and uses drought-tolerant crops.

"There are many uncomfortable realities that mankind is going to face in the context of water as it relates to overuse, land-use change, climate change, that is going to really affect the way we operate," McDonnell says. "Part of the problem is the perception in the U.S. that we have an abundance of water, that it's a right to have unlimited use of water—that it's our right to grow avocados in a water-stressed area, or to green the desert . . . Many of these activities are not sustainable. We can show that with the GRACE data in a way that has been difficult to show clearly. Up until GRACE, the data were anecdotal, or came from individual well observations. GRACE is giving us a more complete picture."